This invention relates to photoanalysis apparatus which measure the amount of resultant light produced by passage of particles in a sample of fluid through a beam of light.
In a photoanalysis apparatus as presently constructed, a fluid sample containing particles to be detected is passed through an optical chamber generally in a thin stream so that the particles pass through the chamber in succession. A beam of light is passed through the chamber in a direction perpendicular to the passage of the fluid sample. When a particle in the fluid sample intersects the light beam, light is scattered and/or fluorescent light is generated. The scattered and/or fluorescent light, hereinafter termed resultant light, may be detected by photodetectors positioned off axis with respect to the beam of light projected through the fluid and perpendicular to the axial direction of flow of the fluid. That is, the source of light and the optical detectors are generally positioned in a common plane but are at angles other than zero with respect to one another.
In the type of apparatus described, a minute amount of resultant light generally is available because the particles to be detected are very small, intersecting only a small amount of light in the light beam. In order to detect the resultant light either a very high intensity light source must be used in order to increase the amount of resultant light produced or very sensitive photodetectors must be employed. In many applications both a high intensity light source such as a laser and very sensitive photodetectors are employed.
High intensity sources which may be employed are expensive and often have a short useful life. Sensitive photodetectors such as noted above, in addition to being very sensitive to small amounts of illumination, are also extremely sensitive to environmental conditions. That is, they respond to a great many different types of noise signals in the same manner as they would respond to very small amounts of illumination. The circuitry employed in association with these highly sensitive photodetectors must be very sophisticated, and therefore very expensive in order to discriminate between signals generated due to noise and signals generated due to low levels of illumination.
A number of arrangements have been employed in order to eliminate the need for very high intensity light sources and very sensitive photodetectors. In one such arrangement, a mirror is located outside of and on one side of the optical chamber, and is aligned with the optical axis of the photodetectors employed. Part of the resultant light produced by passage of a particle through the optical chamber will be reflected by the mirror to the photodetectors thereby increasing the amount and/or intensity of resultant light they receive and detect. In another arrangement, a number of mirrors are positioned outside of the optical chamber and substantially surround the chamber, except, of course for the portions through which the incident beam of light passes. Almost all of the resultant light produced by passage of a particle through the incident beam is reflected by these mirrors. The mirrors are formed such that they will reflect substantially all of the light received in a particular narrow beam onto the photodetectors.
Although the mirror arrangements noted in the preceding paragraph appear to be quite successful in allowing a reduction of the light source intensity and/or sensitivity of the photodetectors employed, they have certain disadvantages. Foremost among the disadvantages appears to be the presence of interfaces which can produce a reduction in light intensity, a distortion of the resultant light, and/or an increase in the noise generated within the system. Examples of interfaces include the interface produced between the fluid in the optical chamber and the material forming the optical chamber; the interface produced by the material formimg the optical chamber and the air between the chamber and the mirror; and the interface formed by all three of the fluid, optical chamber material and air between the chamber and the mirror.